An exemplary apparatus for detecting whether a magnetic element is mounted at a correct position on a retaining member is provided. The apparatus includes a holder, a magnetoresistive sensor, and a processor. The magnetoresistive sensor is held by the holder and capable of sensing a present magnetic flux from the magnetic element to the magnetoresistive sensor. The processor is electrically connected to the magnetoresistive sensor and has a predetermined reference magnetic flux stored information therein, configured for obtaining the present magnetic flux and comparing the present magnetic flux with the reference magnetic flux information, thereby detecting whether a magnetic element is mounted at a correct position on a retaining member.